The present invention relates to method and apparatus for electrolytic treatment in which an electrolytic reaction can be controlled in an optimum manner when a metal plate is subject to an electrolytic surface-roughening treatment. The invention particularly relates to an electrolytic treatment apparatus for electrolyzing an aluminum support or substrate for a printing plate.
An aluminum plate (including an aluminum alloy plate) has conventionally been used as a printing-plate support, particularly as a support or substrate for an offset printing plate. Generally, in order to use an aluminum plate as a support or substrate for an offset printing plate, it is necessary that the aluminum plate have a proper adhesion to a photosensitive material and a proper water-retention property.
To this end, it is necessary to perform surface roughening on an aluminum plate so as to make the aluminum plate have a uniformly and densely grained surface. This surface-roughening treatment has a large influence on the printing performance and printing-durability of a plate material when offset printing is actually performed during use. Accordingly, the quality of the surface-roughening treatment has been an important factor in producing printing plate materials.
As a method for roughening a surface of a support or substrate of an aluminum printing plate, an AC electrolytic etching method has been generally employed. For the current used in this method, an AC current of an ordinary sinusoidal waveform, a current of a special alternating waveform such as a square waveform, or the like has been employed. In this AC electrolytic etching method, surface roughening is effected on an aluminum plate, usually with a single treatment, with an AC current and by using a counter electrode, which may be a suitable electrode such as a carbon electrode or the like. In this method, however, the depth of pits is generally so shallow that the printing-durability has been poor.
Accordingly, there have been proposed various methods for the purpose Of obtaining aluminum plates having grained surfaces in which pits are formed having a depth larger than can be obtained with the above-described methods and are suitable for printing-plate supports or substrates. Among such methods, there is known a surface-roughening method using an electrolytic power source of a special waveform (see Japanese Patent Unexamined Publication No. Sho. 53-67507), a method wherein the ratio of anode time and cathode time is varied in performing electrolytic surface-roughening using an alternating current (see Japanese Patent Unexamined Publication No. Sho. 54-65607), a method wherein the waveform of the power source is varied (see Japanese Patent Unexamined Publication No. Sho. 55-25381), a method wherein a combination of surface current densities are employed (see Japanese Patent Unexamined Publication No. Sho. 56-29699), and the like.
Further, Japanese Patent Publication No. Sho. 61-60797 describes that it is possible to obtain a uniformly roughened surface by application of an alternating waveform voltage having a quiescent time where the voltage is made zero within each period of at least one of the anode time and cathode time to thereby make the current flow such that the quantity of electricity (total charge transferred) during the anode time is larger than that in the cathode time.
In the case of a printing-plate aluminum support made of a material such as a JIS3003, the support contains a large portion of alloy components. However, changes in the shapes of the alloy grains have been sometimes been caused due to small variations in the alloy components between lots or batches of aluminum plates to thereby cause an uneven printing performance. To solve this problem, recently, the present inventors have proposed an electrolytic treatment method for producing an aluminum support or substrate of a printing plate which is characterized in that the time taken for the current to reach its peak value during the anode time and cathode time is made to fall within a range of from 0.1% to 20% (both inclusive) of t.sub.F and t.sub.R, respectively, where t.sub.F and t.sub.R represent the anode time per cycle and the cathode time per cycle, respectively.
To shorten the time taken for the current to reach its peak value, various measures have been attempted, such as reducing the inductive component of the power unit, reducing the inductive component of the load, increasing the resistive component of the load, inserting a resistor in series with the load, and the like.
To reduce the inductive component of the power source or load, the capacity of the power source can be reduced, the size of the electrolytic treatment cell (which is a load) can be reduced, etc. These methods, however, have had a problem in that they are not suitable for mass production.
To increase the resistive component of a load, on the other hand, there has been proposed a method in which a resistor is inserted in series with the load. In this method, however, there has been a problem in that the increase of resistance makes it necessary to increase the power supply voltage, thereby greatly increasing the power cost.
Moreover, there has been a problem that if the load fluctuates, the current waveform is caused to change so as to change the time required for the current to reach its peak value. As a result, the shape of the generated grains is sometimes changed.
In addition, sometimes there has occurred a problem that a gate turn-off thyristor used for generating an alternating current is supplied with a voltage beyond its withstanding voltage so that the power supply fails.